In recent years, in the consideration of global environmental problems, early utilization techniques for new energies have been intensively studied, and fuel cells or batteries have been noticed as one of these techniques. In the fuel cells, hydrogen and oxygen are electrochemically reacted with each other to convert a chemical energy into an electric energy. Thus, the fuel cells are characterized by a high energy utilization efficiency and, therefore, have been positively studied for practical applications to civil life, industries or automobiles. The fuel cells generally known in the art are classified into a phosphoric acid type (PAFC), a molten carbonate type (MCFC), a solid oxide type (SOFC), a solid polymer type (PEFC), etc., according to kinds of electrolytes used therein.
As to the fuel sources for generating hydrogen used in the fuel cells, there have been made various studies on extensive hydrocarbon-containing raw materials including petroleum-based fuels such as kerosene, isooctane and gasoline, LPG or city gases.
As the method of obtaining a reformed gas comprising hydrogen as a main component by reforming the hydrocarbon-containing fuels, there are known various reforming techniques such as SR (steam reforming) method, PDX (partial oxidation) method and SR+PDX (autothermal) method. Among these reforming techniques, application of the steam-reforming (SR) method to cogeneration has been most noticed, since the SR method enables production of a reformed gas having a high hydrogen concentration.
The steam reforming (SR) is conducted according to the following reaction formula:CnH2n+2+nH2O→nCO+(2n+1)H2 CO+H2O→CO2+H2 
In general, the above reaction is conducted at a temperature of 600 to 800° C. and a S/C ratio (steam/carbon ratio) of about 2.0 to about 3.5. In addition, the reaction is an endothermic reaction and, therefore, can be accelerated as the reaction temperature is increased.
In general, in the fuel cell system, there may be used the process in which after a substantially whole amount of sulfur components contained in a fuel is removed therefrom using a desulfurizer, the thus desulfurized hydrocarbon is then decomposed to obtain a reformed gas comprising hydrogen as a main component, and the resulting reformed gas is introduced into a fuel cell stack. In such a conventional process, a reforming catalyst is used to reform the hydrocarbons. However, the reforming catalyst tends to undergo deterioration in catalyst performance during the operation for a long period of time. In particular, the reforming catalyst tends to be poisoned with a trace amount of sulfur components slipped through the desulfurizer, resulting in problems such as significant deterioration in catalytic activity thereof. In addition, when C2 or more hydrocarbons are used as a fuel, the hydrocarbons in the fuel tend to suffer from thermal decomposition, resulting in deposition of carbon on the catalyst, production of polycondensates and deterioration in performance of the reforming catalyst. Also, among these fuel cell systems, the reforming catalysts for PAFC and PEFC are generally used in the form of a molded product such as beads. In this case, if the beads-shaped catalysts suffer from significant coking inside thereof, the catalysts tend to be broken and powdered, resulting in clogging of a reaction tube therewith.
The fuels such as city gases, LPG, kerosene, gasoline and naphtha comprise not only C1 but also C2 or more hydrocarbons. For example, the city gas 13A comprises about 88.5% of methane, about 4.6% of ethane, about 5.4% of propane and about 1.5% of butane, i.e., comprises, in addition to methane as a main component thereof, hydrocarbons having 2 to 4 carbon atoms in an amount as large as 11.5%. Also, LPG comprises about 0.7% of ethane, about 97.3% of propane, about 0.2% of propylene and about 1.8% of butane, i.e., comprises the C4 hydrocarbon in an amount of 1.8%. These C2 or more hydrocarbons tend to be readily thermally decomposed to cause deposition of carbon.
At present, as an active metal species of the steam reforming catalysts, there may be used noble metals such as Pt, Rh, Ru, Ir and Pd, and base metals such as Ni, Co and Fe. Among these metals, in the consideration of high catalytic activity, there have been mainly used catalysts supporting a metal element such as Ni and Ru.
The noble metals such as Ru tend to hardly undergo deposition of carbon even under a low S/C (steam/carbon) ratio condition. However, the noble metals tend to be readily poisoned with sulfur components contained in the raw materials, and deteriorated in catalytic activity for a short period of time. Further, deposition of carbon tends to be extremely readily caused on the sulfur-poisoned catalysts. Thus, even in the case where the noble metals are used, there also tends to arise such a problem that deposition of carbon is induced by the poisoning with sulfur. In addition, since the noble metals are expensive, the fuel cell systems using the noble metals tend to become very expensive, thereby preventing further spread of such fuel cell systems.
On the other hand, since Ni as a base metal element tends to relatively readily undergo deposition of carbon, it is required that the Ni-containing catalyst is used under a high steam/carbon ratio condition in which steam is added in an excessive amount as compared to a theoretical compositional ratio thereof, so that the operation procedure tends to become complicated, and the unit requirement of steam tends to be increased, resulting in uneconomical process. Further, since the conditions for continuous operation of the system are narrowed, in order to complete the continuous operation of the system using the Ni-containing catalyst, not only an expensive control system but also a very complicated system as a whole are required. As a result, the production costs and maintenance costs tend to be increased, resulting in uneconomical process.
Since the steam reforming reaction is a high-temperature reaction and the fuel cell system is subjected to DSS (Daily Start-up and Shutdown) operation, the catalyst body filled in a reactor is gradually closely packed by repeated expansion/contraction and swelling of the reactor owing to external heating, which tends to finally cause breakage of the catalyst. For this reason, in the fuel cell system, α-alumina having a relatively high crushing strength has been generally used as a carrier for the catalyst.
However, the α-alumina has been generally produced by baking a raw material at a high temperature to enhance its crushing strength. Therefore, the resulting α-alumina exhibits an extremely small BET specific surface area and pore volume. As a result, an active metal species supported on the α-alumina tends to be readily sintered when exposed to heat, resulting in deterioration of its catalytic activity.
When using Ni which is relatively susceptible to deposition of carbon as the active metal species, an alkaline element such as CaO and MgO may be added to suppress the deposition of carbon on Ni. However, when the content of the alkaline element is too large, the resulting catalyst tends to be considerably deteriorated in strength.
In addition, in order to suppress the deposition of carbon, if an MgO carrier only is subjected to tablet molding or press molding to thereby forcibly increase a strength of the catalyst, the resulting catalyst tends to be deteriorated in catalytic activity. As a result, it may be very difficult to impart a high activity to the catalyst.
For the above-mentioned reasons, it has been demanded to provide a hydrocarbon-decomposing catalyst which is less expensive and can exhibit as its functions an excellent catalytic activity capable of decomposing and removing hydrocarbons, a good anti-coking property even under a low steam condition, a sufficient strength capable of withstanding crushing and breakage even when coking occurs within the catalyst, and an excellent durability.
Conventionally, there have been reported hydrocarbon-decomposing catalysts formed by supporting a catalytically active metal such as platinum, palladium, ruthenium, cobalt, rhodium, ruthenium and nickel on a carrier comprising α-alumina, magnesium oxide or titanium oxide (Patent Documents 1 to 4, etc.). Also, there are known the methods for producing a hydrocarbon-decomposing catalyst by using an Ni-containing hydrotalcite compound as a precursor (Patent Documents 5 to 7, etc.)
Patent Document 1: Japanese Patent Application Laid-Open (KOKAI) No. 9-173842
Patent Document 2: Japanese Patent Application Laid-Open (KOKAI) No. 2001-146406
Patent Document 3: Japanese Patent Application Laid-Open (KOKAI) No. 2004-82034
Patent Document 4: Japanese Patent Application Laid-Open (KOKAI) No. 2003-284949
Patent Document 5: Japanese Patent Application Laid-Open (KOKAI) No. 2000-503624
Patent Document 6: Japanese Patent Application Laid-Open (KOKAI) No. 2003-135967
Patent Document 7: Japanese Patent Application Laid-Open (KOKAI) No. 2004-255245